1. Field of the Invention
This invention relates to an element for generating near-field light, a head for a high-density information recording device making use of it, and a probe for a high-resolution microscope.
2. Description of the Related Art
Near-field light-generating elements are used in optical heads within optical recording devices for making high-density information recordings and in optical probes within near-field optical microscopes for making observations at high resolutions.
As amounts of information of still images and moving images have increased explosively in recent years, high-density optical recording devices have been developed actively. It is known that optical disks typified by compact discs (CDs) have limited recording densities due to diffraction limit of light. To exceed this limitation, a method utilizing a shorter wavelength of light and a method making use of near-field light have been proposed. An optical recording device using near-field light is a method consisting of causing light to enter an optically small aperture having a subwavelength size, causing the near-field light spreading a little past the aperture to interact with the surface of the recording medium, and detecting scattered light transmitted or reflected to thereby read out microscopic data marks. Since the minimum mark size capable of being recorded and read is limited not by the wavelength of the incident light but by the size of the aperture, the recording density can be enhanced by fabricating a microscopic aperture.
In an optical recording device employing nearfield light, the aperture is required to be placed close to the surface of the recording medium. Furthermore, to achieve a high data transfer rate, the aperture needs to scan over the surface of the recording medium at a high speed. To satisfy these requirements, a flying head method similar to that used in conventional magnetic recording has been proposed (Issiki, F. et al. Applied Physics Letters, 76(7), 804 (2000)). The head is fabricated by forming a floating slider and a minute aperture on a planar substrate by semiconductor processes. For example, a SiO2 layer is laminated on a Si substrate. A resist pattern for a tip is formed by lithography. The SiO2 layer is etched to fabricate the conical tip made of SiO2. Al is deposited to 200 nm by vacuum evaporation and then the front end of the tip is cut by the FIB (focused ion beam) method. As a result, a tip having an optical aperture at its front end is fabricated. The contour shape of the aperture is determined by the shape of the resist pattern for the tip. To fabricate a microscopic aperture finally, the contour is preferably circular or rectangular. However, a rectangle is not desirable because there is the possibility that the front end becomes like a blade. Where the aperture shape is a circle, it is not necessary to control the direction in handling the head subsequently. Therefore, a circular aperture is normally formed.
An optical probe used in a near-field optical microscope is fabricated by heating, drawing, and cutting an optical fiber, depositing a light-shielding film of Al, and then cutting the front end to form an optical aperture.
Incident light from a laser light source is directed to the aforementioned optical head or probe to thereby produce near-field light. The incident light is guided from the laser by an optical fiber and propagated through air to the microscopic aperture. The light from the laser is linearly polarized light. When the light is being guided by the fiber, the polarization is disturbed. When the light is propagated through air, it is unlikely that the device is so operated that the shape of the aperture, the scanning direction, and the direction of polarization are controlled.
The problem with the aforementioned near-field optical probe or head is that the intensity of near-field light (herein referred to as the light efficiency of the probe) generated from the aperture is small compared with the intensity of the incident light. The incident light is reflected off the inner wall of the probe or absorbed before the light reaches the aperture. Thus, the light is lost as thermal energy. Even with respect to the light reaching the aperture, only small energy can pass through, because the aperture size is smaller than the wavelength. If the intensity of the generated near-field light is weak, sufficient contrast cannot be obtained. In the case of a microscope, the accuracy of the output image will be insufficient. In the case of a data storage device, the data transfer rate will be insufficient.
Contrivances have been made to improve the light efficiency, for example, in Veerman, J. A. et al., Applied Physics Letters, 72(24), 3115 (1998), where the front end of a probe is cut by FIB, the beam is directed to the probe from just beside it to flatten the front end. Conversely, in Ohtsu, M., J. Lightwave Tech., 13(7), 1200 (1995), an attempt is made to improve the resolution by forming a microscopic protrusion within a plane of an aperture.
However, it is known that where the size of the aperture is reduced to improve the resolution of the microscope or the recording density of the storage device, the light efficiency deteriorates. A method of improving the light efficiency is being explored.